Functional characterization of a novel flavonoid glycosyltransferase UGT89AD6 from Anemarrhena asphodeloides Bunge and mechanistic insights into its enzymatic O-glycosylation.

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conclusionUGT89AD6 is a versatile O-glycosyltransferase with broad substrate and sugar donor promiscuity, offering a valuable biocatalyst for flavonoid glycoside synthesis. Anemarrhena asphodeloides Bunge, known as a famous monocotyledonous medicinal plant in China, contains a typically important secondary metabolite icariside I with various pharmacological activities. In A. asphodeloides, icariside I is derived from the O-glycosylation of icaritin. However, the O-glycosyltransferase from A. asphodeloides catalyzing the glycosylation of icaritin remains largely unidentified. Herein, a promiscuous O-glycosyltransferase UGT89AD6 from A. asphodeloides was successfully identified and characterized. It exhibited catalytic activity toward icaritin, thereby forming the glycosylated product icariside I. It also showed broad glycosylation activities toward other structurally diverse substrates, including eight other flavonoid compounds and an anthraquinone scaffold. Furthermore, it could accept four UDP-sugars as the donors; the preference order was UDP-Xyl > UDP-Glc > UDP-Rha > UDP-Ara. Structural analysis indicated that the broad O-glycosylation activities of UGT89AD6 could be linked to the unique spacious substrate binding pocket. Beyond this, the regioselectivity of UGT89AD6 might be shaped by hydrophobic interactions and hydrogen-bonding network, as well as shallow cavity of the binding pocket. This study contributes to the diversity of glycosyltransferases from monocotyledonous plants and provides a potential biocatalyst to synthesize high-value bioactive flavonoid glycosides.